gitlab-ci: Use source-package-builder image as package-builder is deprecated

The package-builder image is a multi-gigabytes image with a bunch of random packages pre-installed. It originated as a replacement for the specially configured Jenkins workers used in the past to build a very restricted set of packages and it is specifically tailored to ship all their build-dep out of the box as the Jenkins jobs ran as unprivileged users so they could not install them on their own.